A high-performance sports car normally has a rear-wheel drive, and a self-locking differential to maximize torque transmission by the rear wheels to the road surface in any driving condition.
A rear-wheel drive with a self-locking differential has advantages when driving in racing mode in good (dry-road) or fairly good (wet-road) grip conditions, but, in conjunction with high torque and wide tyres, makes for extremely difficult, potentially hazardous driving in poor grip conditions (flooded or icy roads). To improve handling of sports cars in poor grip conditions, a permanent or optionally connectable four-wheel drive has been proposed.
A permanent four-wheel drive greatly improves vehicle performance in poor grip conditions, but has the drawback of permanently increasing torque loss of the drive system and so resulting in good-grip vehicle performance which is not always popular with drivers.
An optionally connectable four-wheel drive, on the other hand, enables the driver to switch between a rear-wheel drive in good grip conditions, and a four-wheel drive in poor grip conditions.
Patent Application WO2005080117A2 describes an optionally connectable four-wheel drive vehicle comprising an engine with a drive shaft; two main drive wheels connected permanently to the drive shaft via a transmission with a first clutch; and two secondary drive wheels selectively connectable to the drive shaft by an optionally connectable drive system having a second clutch which is connected on one side with a fixed velocity ratio to the drive shaft upstream from the transmission, and is connected on the other side with a fixed velocity ratio to the secondary drive wheels.
Patent Application WO2006100585A1 describes a refinement of the optionally connectable four-wheel drive vehicle in Patent Application WO2005080117A2. More specifically, the percentage value of the torque to be transmitted to the secondary drive wheels by the second clutch is determined instant by instant as a function of dynamic vehicle parameters detected by respective sensors.
In the solutions proposed in Patent Applications WO2005080117A2 and WO2006100585A1, the four-wheel drive can only be engaged in low gears (typically, first, second, and third), and not at all in the other gears. Patent Application WO2008120097A1 describes a refinement of the optionally connectable four-wheel drive vehicle in Patent Applications WO2005080117A2 and WO2006100585A1, in which the optionally connectable secondary power train for connecting the drive shaft to the secondary drive wheels comprises a gear drive with at least two different, alternatively selectable velocity ratios. In this way, the optionally connectable secondary power train in Patent Application WO2008120097A1 is engageable (i.e. permits torque transfer to the front wheels) in substantially all the gears of the transmission.
Though, none of the connectable four-wheel drives described in the above mentioned patent applications is engageable (i.e. permits torque transfer to the front wheels) when the reverse gear is selected; this limitation can cause problems for the driver, since the reverse gear can be used to steer the car in bad drive conditions (e.g. to drive in reverse along a wet or icy ramp).
U.S. Pat. No. 1,853,058A1 describes a four-wheel drive vehicle comprising: an engine presenting a drive shaft; a pair of rear driving wheels; a rear power train connecting the drive shaft to the rear driving wheels and comprising a rear electromagnetic clutch, an electric motor, a rear transmission and a rear differential; a pair of front driving wheels; and a front power train connecting the drive shaft to the front driving wheels and comprising a front electromagnetic clutch, a front transmission, and a front differential. The front transmission and the rear transmission are controlled in a common and synchronized way by means of a mechanical interconnection and comprise a plurality of forward gears; though, the front and rear transmissions are not provided with a reverse gear.
Overview
Embodiments disclosed herein provide an optionally connectable four-wheel drive vehicle that is inexpensive and easy to produce, eliminates the aforementioned drawbacks, and at the same time provides for improving performance, stability, and drive in all grip conditions.
According to the embodiments of the present disclosure, there is provided an optionally connectable four-wheel drive vehicle, as claimed in the attached claims.